We present Spitzer mid-infrared imaging of a sample of 35 tidally distorted premerger interacting galaxy pairs selected from the Arp Atlas. We compare their global mid-infrared properties with those of normal galaxies from the SINGS Spitzer Legacy survey, and separate the disk emission from that of the tidal features. The ½8:0 m À ½24 m, ½3:6 m À ½24 m, and ½5:8 m À ½8:0 m colors of these optically selected interacting galaxies are redder on average than those of spirals, implying enhancements to the mass-normalized star formation rates (SFRs) of a factor of $2. Furthermore, the 24 m emission in the Arp galaxies is more centrally concentrated than that in the spirals, suggesting that gas is being concentrated into the inner regions and fueling central star formation. No significant differences can be discerned in the shorter wavelength Spitzer colors of the Arp galaxies compared to the spirals, and thus these quantities are less sensitive to star formation enhancements. No strong trend of Spitzer color with pair separation is visible in our sample; this may be because our sample was selected to be tidally disturbed. The tidal features contribute 10% of the total Spitzer fluxes on average. The SFRs implied for the Arp galaxies by the Spitzer 24 m luminosities are relatively modest, $1 M yr À1 on average.

We present new three‐dimensional, hydrodynamic simulations of the ram pressure stripping of disc galaxies via interaction with a hot intracluster medium (ICM). The simulations were carried with the smoothed‐particle hydrodynamics, adaptive mesh ‘hydra’ code (SPH‐AP3M), with model galaxies consisting of gas and stellar disc components and dark haloes. The simulations also include radiative cooling, which is important for keeping the warm, diffuse gas of moderate density from being unrealistically heated by the ICM. We examine the role that wind velocity, density and galaxy tilt play in gas stripping. We include cases with lower ram pressures than other recent studies. In accord with previous studies, we find that low column density gas is promptly removed from the outer disc. However, we also find that not all of the gas stripped from the disc escapes immediately from the halo, some of material can linger for times of order 108 yr. We use a simple analytic model to demonstrate that gas elements in the ICM wind feel an effective potential with a minimum displaced downstream from the halo centre. The onset of the ICM wind has a profound effect on the disc gas that is not immediately stripped. This remnant disc is displaced relative to the halo centre and compressed. This can trigger gravitational instability and the formation of numerous flocculent spirals. These waves transport angular momentum outward, resulting in further compression of the inner disc and the formation of a prominent gas ring. This ‘annealing’ process makes the inner disc, which contains much of the total gas mass, resistant to further stripping, but presumably susceptible to global starbursts. Spirals in the outer disc stretch, shear and are eventually stripped on time‐scales of a few times 108 yr, after which time, mass and angular momentum loss effectly cease. For inclined galaxies, these effects are considerably modified over the same time‐scale. The amount of mass loss is reduced. In addition, we find that a higher galaxy tilt couples the wind and the rotating disc, and produces a higher degree of angular momentum removal. Temperature and line‐of‐sight velocity maps from several of the simulations are presented for comparison with observation. When the mass loss and annealing processes go to completion, we find that the total amount of mass lost from a fixed target galaxy is well‐fitted by a simple power‐law function of a dimensionless parameter that combines the ram pressure and internal properties of the galaxy. Ramifications for the cluster galaxy evolution are discussed.

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Galaxy mergers are believed to trigger strong starbursts. This is well assessed by observations in the local Universe. However the efficiency of this mechanism has poorly been tested so far for high redshift, actively star forming, galaxies. We present a suite of pc-resolution hydrodynamical numerical simulations to compare the star formation process along a merging sequence of high and low z galaxies, by varying the gas mass fraction between the two models. We show that, for the same orbit, high-redshift gasrich mergers are less efficient than low-redshift ones at producing starbursts: the star formation rate excess induced by the merger and its duration are both around 10 times lower than in the low gas fraction case. The mechanisms that account for the star formation triggering at low redshift -the increased compressive turbulence, gas fragmentation, and central gas inflows -are only mildly, if not at all, enhanced for high gas fraction galaxy encounters. Furthermore, we show that the strong stellar feedback from the initially high star formation rate in high redshift galaxies does not prevent an increase of the star formation during the merger. Our results are consistent with the observed increase of the number of major mergers with increasing redshift being faster than the respective increase in the number of starburst galaxies.